Apparatus and method for improved energy dispersive X-ray spectrometer

ABSTRACT

The invention is an improved energy dispersive x-ray spectrometer comprising x-ray source ( 10 ), sample ( 12 ), optics ( 20 ) and detector ( 16 ). The improvement comprises the use of optics using the principle of total external reflection for delivering an increased flux of x-rays onto the detector. These flux concentrating optics are generally shaped as tubes having a figure of revolution as a longitudinal cross section such as a cone, parabola, hyperbola, ellipsoid and others. These optics may also be used as low energy pass filters by incorporating a stop ( 22 ) at or near the aperture of the optic. The reflecting surface of the flux concentrator optic may be selected from the group of metals and their alloys, the choice made to optimize the optic&#39;s performance.

This application claims benefit of provisional application 60/055,829filed Aug. 15, 1997.

BACKGROUND

1. Field of Invention

This invention relates to the field of x-ray spectrometry and otherinstrumentation relying on energy dispersive spectrometry analysis ofx-rays emitted from a target, and specifically to a method and apparatusfor preserving the flux of x-rays emitted from said target and availablefor analytical purposes within the instrument.

2. Description of Prior Art

In x-ray spectroscopic elemental analyses, there are two main techniquesfor separating into their various energies the x-rays emitted from thesample under analysis, these are wavelength dispersive spectroscopy(WDS) and energy dispersive spectroscopy (EDS).

WDS uses Bragg reflection from a crystal to separate the x-rays intovarious wavelengths, while EDS uses a solid state device whose output isa known function of the x-ray energy to separate them into variousenergy bins. For reasons of mechanical simplicity, data collectiontimes, spectrometer size and various other technical reasons, EDS isfrequently preferred over WDS. Unfortunately, EDS systems are frequentlyinefficient at detection of very low energy x-rays and this inefficiencyis exacerbated by the low production rate of low energy x-rays comparedto higher energy ones. These low energy x-rays correspond to thoseemitted by light elements such as Be, B, C, N, O, and F and also theso-called L lines of the heavier elements. Because of the low productionrate of these low energy x-rays compared to higher energy ones, an EDSsystem spends most of its time counting pulses from higher energyx-rays, thus adversely affecting the instrument's detection limit forthe light elements, thereby making these elements detectable by EDS onlywhen found in relatively high concentration in the sample. The x-raylines for the light elements are closer in energy than those for theheavier elements and many EDS systems have difficulty resolving thesex-ray lines when L lines from heavier elements are also emitted by thesample. EDS cannot be used for light element analyses except in specialcases or if the EDS detector is specially configured for optimumresolution. Resolution is often a decreasing function of detector size,because a smaller detector has less capacitance than a larger one. Also,a small detector subtends a smaller collection angle than a larger one,resulting in a lower count rate. In some cases, the size of the detectoris limited by the instrument's geometry, so that a smaller detector isnecessary even when poor resolution occurs. For example, some electronmicroscopes have sufficiently close working distances or otherwise pooraccess to x-rays emitted from a sample, that either a very smalldetector must be used or the detector must be placed far from the x-raysource.

OBJECTS AND ADVANTAGES

Accordingly, several objects and advantages of the present inventionare:

a) to solve the practical problem of increasing the efficiency of x-raydetection in an energy dispersive x-ray spectrometer by gathering x-raysthat are diverging away and redirecting them through an aperture;

b) to provide an energy dispersive x-ray spectrometer which collectsdata faster than existing systems;

c) to provide an energy dispersive x-ray spectrometer which is moresensitive than previous instruments,

d) to teach a method to limit the area of the target from which x-raysreach the detector, thereby allowing the analysis of very small areaswithin a sample.

DESCRIPTION OF DRAWINGS

FIG. 1. shows an X-Ray source with x-rays being emitted into a largesolid angle, some of them reaching the detector.

FIG. 2. shows the same detector with a flux concentrating optic of sometype redirecting captured x-rays, which would otherwise be missed, ontothe detector surface.

FIG. 3 is a graph of reflectivity as a function of photon energy for anickel reflector.

FIG. 4 illustrates one embodiment of the optic being used with an energydispersive x-ray spectrometer to capture and guide the x-ray flux to thedetector.

FIG. 5 shows a conical optic with a double taper being used to collectan x-ray flux to direct to a detector.

FIG. 6 shows an out of focus paraboloid optic used to concentratediverging x-rays onto a detector in an energy dispersive x-rayspectrometer.

LIST OF REFERENCE NUMERALS

Incident energy beam

12 Target sample

14 Emitted isotropic x-rays

16 Detector

18 Redirected x-rays

20 Optic

22 Aperture stop

24 Entrance aperture

26 Exit aperture

28 Paraboloid focus

SUMMARY

This invention teaches a method and apparatus for directing a largerflux of x-rays onto the x-ray detector in an energy dispersivespectrometer, thereby allowing the detector to see more x-rays. Byproviding for a larger stream of x-rays falling onto the detector, thisinvention allows faster data collection and, thereby, greatersensitivity in the instrument. In addition, by teaching a method andapparatus allowing the selective isolation of x-rays emitted from verysmall areas of a source sample, the invention allows the analyticaldifferentiation and localization of elements within a composite sample.

DESCRIPTION OF THE INVENTION

The invention disclosed in this patent application concerns a method ofimproving an energy dispersive x-ray spectrometer (EDS) through the useof a reflecting optic 20 by collecting through an entrance aperture 24the isotropic x-rays 14 emitted from the target sample 12 and guidingthose redirected x-rays 18 through an exit aperture 26 onto the x-raydetector 16. The optics 20 herein described, when fitted with a stop 22at or near the entrance aperture 24, may also be used in the EDS aslow-pass filters that reduce the number of higher energy x-rays reachingthe detector 16 and may be used as well to spatially define the area ofthe target sample 12 from which emitted x-rays 14 are selected foranalysis.

The optics 20 described in this application are specifically of the typeknown as “grazing incidence optics”, which use the principle of totalexternal reflection for the efficient collection of x-rays. Grazingincidence optics have been used for other x-ray spectroscopyapplications, notably in three other co-pending patent applications bythis inventor (U.S. Application Nos. 543,170 and 797,199; andinternational application No. PCT/US96/16234, which was published asWO97/14156. However, the optics described herein, in contrast to theearlier disclosures, are not necessarily collimators or focusers but cansimply be described as flux concentrators, although more accurately theyact as flux preservers.

These optics 20 solve the problem of gathering emitted isotropic x-rays14 that are diverging into a large angle guiding the redirected x-rays18 through an aperture. This aperture is the surface of the detector 16,therefore, it is immaterial where the redirected x-rays 18 pass throughthe aperture. Because the redirected x-ray flux merely has to passthrough the detector aperture and it is not necessary that it be in theform of a parallel beam, the optic does not need to be a collimator.These optics 20 may, therefore, be very simple and their geometry andsurface roughness requirements are less stringent, compared to presentlyavailable systems, thus making these collection optics inexpensive toproduce. For visible light, the capture illustrated in FIG. 2 would beunremarkable, but for x-rays the fabrication of such reflecting opticshas been so problematic that they have not been previously considered.It is the difficulty of fabricating x-ray optics which has kept othersfrom considering such optics as applied herein, thus, most peoplebelieve that these x-ray optics are not practical.

In addition, these optics also solve the problem of limiting the area ofthe target sample 12 from which x-rays reach a detector 16. For example,in x-ray fluorescence, an incident energy beam 10 consisting of x-raysis used to irradiate a large area of the target sample 12. The samplewill become excited by the incident x-rays and, in turn, emit isotropicx-rays 14 which are then read by a detector 16 and analyticallycharacterized to provide sample identification. Most samples 12,however, are not homogeneous and will emit x-rays which vary in spectrumaccording to the molecular composition of each microlocality within thesample. By limiting the area of the target sample 12 seen by thedetector, the redirected x-rays 18 may be characterized to yield spatialresolution of the composition of the target sample 12. Currently, thisis accomplished by placing a small aperture very close to the x-rayemitting sample, so that only those x-rays passing through the aperturewill reach the detector. However, the disadvantage of this approach isthat most of the x-rays passing through the aperture do not reach thedetector because of the distance between the aperture and the detector,rather, the x-rays scatter and only few reach the detector. The optics20 described herein may be used in place of the aperture, not only toincrease the x-ray flux reaching the detector but also to further narrowthe area of target sample 12 seen by the detector, thereby. surpassingpresent methods by providing both increased x-ray flux and increasedspatial resolution of the sample.

Several reflector shapes provide sufficient flux gain to be useful invarious applications. The term “flux gain” is used herein to mean theincrease in x-rays available to the detector 16 in an EDS instrumentfitted with optic 20, as compared to an instrument without the optic.The flux concentrating optics 20 disclosed herein are figures ofrevolution, comprising sections such as cones, parabolas, hyperbolas,ellipsoids and others. Also effective as flux concentrating optics 20are more complex figures of revolution embodying a plurality of any oneshape or combining more than one shape in a single reflector, forexample, composite reflectors comprising conical and/or paraboloidaland/or elliptical and/or hyperbolic reflecting surfaces in addition, forexample, the optic could be fashioned as an out-of-focus paraboloid,FIG. 6, where the paraboloid focus 28 is essentially not coincident withthe x-ray source. While all these reflector shapes are suitable as fluxconcentrating optics 20, the specific reflector geometry chosen for aparticular application will depend on various considerations, includingdesired flux gain, source of the x-rays to be analyzed workingdistances, size of the aperture through which the x-rays must be guided,desired area of the x-ray emitting material to be covered, and others.

The optical fabrication process developed by this inventor and claimedfor collimator construction in co-pending U.S. patent application No.08/543,170, and co-pending international application No. PCT/US96/16234,incorporated herein by reference, is ideally suited for fabrication offlux concentrating optics 20. However, there are known in the art manyother ways of fabricating these optics in addition to the methoddescribed in the above referenced patent application, including epoxyreplication of molds, galvanoforming, controlled shaping of internalsurfaces, bending of smooth sheets of material and others.

OPERATION OF THE INVENTION

Those skilled in the art will appreciate that this invention provides animprovement that was previously unavailable in energy dispersive x-rayspectrometry instrumentation. The operation of the invention isdependent upon reflectors 20 which collect a segment of the availableflux of x-rays emitted 14 by a target sample 12 and redirects the x-rays18 to the instrument's detector device 16.

Grazing incidence optics reflect x-rays only for very small angles withrespect to the reflecting surface (FIG. 3), and the reflectivity is astrongly decreasing function of grazing angle and x-ray energy.Reflectivity is also a strong function of the type of reflectingsurface, a metal or metal alloy, and various surfaces may be chosen tomatch the x-rays of interest. In most grazing incidence x-ray optics,scatter from reflecting surfaces due to micro-roughness is a seriousproblem, but in this case it is less severe and can even be used toadvantage. In most x-ray optical systems, the accuracy of the surfacefigure of the reflecting surface is critical in guiding the reflectedx-rays in the desired direction but in this case, small variations willstill allow the x-rays go through the detector, therefore makingaccuracy less important.

Referring to FIG. 2, with no losses on reflection the flux gain in ananalytical instrument fitted with such an optic 20, relative to nooptic, would be given by the formula

G=I _(o) /I _(d)

where I_(d) is the solid angle subtended by the detector and I_(o) isthe solid acceptance angle of the optic. If the angle subtended by thedetector is small, a relatively small increase in effective acceptanceangle, due to the acceptance angle of the optic, can give a large gain.Even where these optics produce a small gain, they may provide enough ofan improvement in a particular analytical system to be consideredsignificant.

Because of the strong decrease of reflectivity with increasing grazingangle, it is desirable to minimize the bend angle of an x-ray to make itgo through the detector aperture. The minimum bend angle for an x-ray isthe angle which makes it pass into the outer edge of the detector,although greater bend angles will work with reduced reflectivity. Forcases where the bend angle is so large that it would result inunacceptably low reflectivity, multiple smaller bend angles may be used,taking advantage of the fact that sometimes several smaller reflectionsgive better total reflectivity than a single large bend. This isparticularly true for higher x-ray energies.

If the entrance aperture 24 of the optic is blocked by a stop 22, sothat no direct x-rays reach the detector, only the lower energyreflected x-rays will reach it and the optic effectively becomes a lowenergy pass filter. The grazing angles and reflecting surface may bechosen specifically to determine the energy passed by such an optic. Inaddition, optics of this type effectively limit the accepted x-ray fluxto that emitted from a small area so that, even if a large area isradiating, the user can be assured that the x-rays originate from aspecific small area.

In summary, this invention teaches an improvement in energy dispersivex-ray spectrometry through the use of optics using the principle oftotal external reflection for delivering an increased flux of x-raysonto the detector. These flux concentrating optics are generally shapedas tubes having as longitudinal cross section a figure of revolution,such as cones, parabolas, hyperbolas, ellipsoids and others. Alsodisclosed as effective flux concentrating optics are more complexfigures of revolution embodying a plurality of any one shape orcombining more than one shape in a single reflector, for example,compound conical reflectors, compound parabolic reflectors, compoundelliptical reflectors and compound hyperbolic reflectors, as well asoff-axis and out-of-focus figures of revolution. These optics may alsobe used as low energy pass filters. The reflecting surface of the fluxconcentrator optic may be selected from the group of metals and theiralloys, the choice made to optimize the optic's performance. Theimproved EDS may be incorporated in instruments having otherapplications, including transmission electron microscopy, scanningelectron microscopy, electron microprobes, x-ray fluorescenceinstrumentation, and may also be used for inspection of semiconductorvias. The improved EDS also provides increased spatial resolution of thesample, as described herein above.

What is claimed is:
 1. An energy dispersive x-ray spectrometercomprising: a) a beam source generating a beam of exciting energydirected at a target to thereby cause said target to emit x-rays; b) areflector for reflecting said emitted x-rays, said reflector having areflector body generally shaped in the form of a tube having an innercavity, an entrance aperture positioned for providing an entranceopening into said inner cavity, an exit aperture positioned forproviding an exit opening from said inner cavity, said inner cavitycomprising a seamless surface of x-ray reflective material forreflecting said emitted x-rays, said reflector positioned substantiallyadjacent said target to collect emitted x-rays through the entranceaperture and reflect said x-rays from the x-ray reflecting surfacethrough the exit aperture; and c) a detector having a detector aperturepositioned adjacent the exit aperture of said reflector for detectingreflected x-rays to thereby generate data corresponding to said x-rays.2. The energy dispersive x-ray spectrometer of claim 1, wherein saidreflector includes an entrance aperture having an area smaller than saidtarget to thereby collect x-rays emitted by generally only apredetermined area of said target.
 3. The energy dispersive x-rayspectrometer of claim 1, wherein said x-ray reflecting surface furthercomprises at least one predetermined x-ray reflective material selectedfrom metals and metal alloys.
 4. The energy dispersive x-rayspectrometer of claim 1, wherein said reflector further comprises ashape of at least one figure of revolution.
 5. The energy dispersivex-ray spectrometer of claim 1, exciting energy comprises substantiallyparallel x-rays.
 6. The energy dispersive x-ray spectrometer of claim 1,further comprising a stop positioned relative to said reflector so as tothereby allow generally only reflected x-rays to pass through the exitaperture.
 7. The energy dispersive x-ray spectrometer of claim 1,wherein said beam of exciting energy comprises energy selected fromelectrons, and x-rays.
 8. A method for energy dispersive x-rayspectrometry, the method comprising the steps of: a) irradiating atarget with a beam of exciting energy sufficiently to thereby cause thetarget to emit x-rays; b) collecting emitted x-rays in a reflectorhaving a reflector body generally shaped as a tube having a first endand a second end, an inner cavity positioned between the first end andthe second end, an entrance aperture positioned generally at the firstend to connect with the inner cavity thereby providing an entranceopening into the inner cavity, an exit aperture positioned generally atthe second end to connect with the inner cavity thereby providing anexit opening from the inner cavity, a seamless x-ray reflecting surfacefor reflecting the x-rays, the reflector positioned adjacent the targetto collect the emitted x-rays through the entrance aperture and receivethe emitted x-rays on the x-ray reflecting surface to thereby reflectthe x-rays through the exit aperture; and c) detecting the reflectedx-rays in a detector having a detector aperture positioned adjacent theexit aperture of the reflector for collecting substantially all saidreflected x-rays.
 9. The method of claim 8, wherein collecting comprisesemitted x-rays from substantially a predetermined area of said target.10. The method of claim 8, wherein the beam of exciting energy comprisesenergy selected from electrons, x-rays, and substantially parallelx-rays.
 11. The method of claim 8, wherein the collecting furthercomprises allowing generally only reflected x-rays to pass through theexit aperture.
 12. The method of claim 8, wherein the x-ray reflectingsurface further comprises a shape of at least one figure of revolution.13. The method of claim 8, wherein the x-ray reflecting surface furthercomprises at least one predetermined x-ray reflective material selectedfrom metals and metal alloys.
 14. An energy dispersive x-rayspectrometer comprising: a) a source of incident x-rays; b) a targetreceiving said incident x-rays so as to generate emitted x-rays fromsaid target responsive to said incident x-rays; c) a detector forreceiving and quantifying x-rays emitted by said target; and d) areflector generally shaped as a tube having an entrance aperture and anexit aperture, said reflector further comprising an x-ray reflectingseamless surface shaped as a figure of revolution for reflecting x-raysby grazing incidence, said reflector being positioned sufficiently closeto said target to collect said emitted x-rays through said entranceaperture and receive said emitted x-rays on said reflecting surface,thereby to reflect and to discharge said emitted x-rays from said exitaperture, whereby said emitted x-rays are redirected onto the detector.15. The energy dispersive x-ray spectrometer of claim 14, wherein saidreflecting surface further comprises a predetermined material selectedfrom the group consisting of metals and metal alloys.
 16. The energydispersive x-ray spectrometer of claim 15, wherein the shape of saidreflecting surface further comprises a plurality of figures ofrevolution.
 17. The energy dispersive x-ray spectrometer of claim 16,wherein said reflector further comprises a stop at said entranceaperture.
 18. The energy dispersive x-ray spectrometer of claim 15,wherein said reflector further comprises a stop at said entranceaperture.
 19. A method for improved energy dispersive x-rayspectrometry, said method comprising the steps of: a) striking a targetwith a beam of incident x-rays; b) emitting x-rays from the target inresponse to striking; c) receiving the emitted x-rays in a reflectorgenerally shaped as a tube having an entrance aperture and an exitaperture, the reflector further comprising an x-ray reflective seamlesssurface shaped as a figure of revolution for reflecting the emittedx-rays by grazing incidence, the reflector being positioned sufficientlyclose to the target to receive a substantial fraction of the emittedx-rays through the entrance aperture; and d) reflecting the emittedx-rays from the x-ray reflective surface such that the emitted x-raysthereby emerge from the exit aperture redirected onto a detector. 20.The method of claim 19, wherein said reflecting surface furthercomprises a predetermined material selected from the group consisting ofmetals and metal alloys.
 21. The method of claim 20, wherein the shapeof said reflecting surface further comprises a plurality of figures ofrevolution.
 22. The method of claim 21, wherein said reflector furthercomprises a stop at said entrance aperture.
 23. The method of claim 20,wherein said reflector further comprises a stop at said entranceaperture.
 24. A method for energy dispersive x-ray spectrometry, themethod comprising the steps of: irradiating a target with energy so asto cause the target to emit x-rays; and reflecting the emitted x-raysalong a seamless x-ray reflecting surface generally shaped as a tube tothereby convey the x-rays toward a detector for generating datacorresponding to the x-rays.
 25. The method of claim 24, whereinreflecting comprises at least one predetermined x-ray reflectivematerial selected from metals and metal alloys.
 26. The method of claim24, wherein the reflecting surface comprises at least one figure ofrevolution.
 27. The method of claim 24, wherein reflecting comprisesblocking substantially all but reflected x-rays from passing through thex-ray reflector.
 28. The method of claim 24, wherein irradiatingcomprises energy selected from electrons, and x-rays.
 29. The method ofclaim 24, wherein irradiating includes energy comprising a beam ofsubstantially parallel x-rays.
 30. The method of claim 24, furthercomprising collecting x-rays emitted substantially from a predeterminedarea of the target.
 31. An x-ray spectrometer comprising: a source ofx-rays; an energy dispersive x-ray detector including a detector surfaceand a generally planar detector entrance aperture having an area largerthan said source; and an optic including a seamless x-ray reflectivesurface having an optical axis of cylindrical symmetry, an entranceaperture, and an exit aperture to thereby provide within the optic totalexternal reflection of the x-rays; wherein the entrance aperture of saidoptic is positioned relative to said source so as to collect and reflectx-rays toward the detector entrance aperture along a plurality of anglesof incidence relative to the generally planar detector entrance apertureso that substantially the entire generally planar detector entrance isilluminated by x-rays.
 32. The x-ray spectrometer of claim 31, whereinsaid optic collects and reflects x-rays toward the detector entranceaperture along a plurality of divergent angles of incidence relative tothe generally planar detector entrance aperture.
 33. The x-rayspectrometer of claim 31, further comprising a stop positioned relativeto said optic so as to allow generally only reflected x-rays to passthrough the optic exit aperture.
 34. The x-ray spectrometer of claim 31,wherein said optic further comprises a plurality of optics.
 35. Thex-ray spectrometer of claim 31, wherein said optic further comprises aplurality of optics arrayed along a common optical axis.